1. Field of the Invention
The present invention relates to a voltage controlled oscillator (called hereinafter a "VCO") and, more particularly, to a VCO which is free from adjustment of a free-running oscillating frequency.
2. Description of the Prior Art
A VCO has been widely used for various systems. The VCO has a free-running oscillating frequency which should be adjusted to match the operation of the system in which the VCO is used, such as a PLL (Phase Locked Loop). For such usage, it is preferable for the free-running oscillating frequency to be automatically set to a center frequency of a capture range of the PLL circuit without any adjustment. In the prior art, the VCO used in a PLL circuit employs a resonance circuit consisting of a resistor and a capacitor, the value of the resistor and/or the value of the capacitor being adjusted to bring the free-running oscillating frequency into conformity with the center frequency. Such an adjustment requires considerable man-hours and time. Therefore, a VCO which does not require the free-running oscillating frequency adjustment has been widely demanded.
One such adjustment free VCO in the prior art is shown in FIG. 1 in a block diagram. This VCO includes an amplifier 8, a ceramic resonator 10 and a variable reactance circuit 11. The output of the amplifier 8 is positively fed back to the input thereof, and further connected to the ceramic resonator 10 and the variable reactance circuit 11. The output of the amplifier 8 is led out from an output terminal 9 of the VCO. The equivalent capacitance of the variable reactance circuit 11 is changed in accordance with a control voltage applied thereto via a control terminal 12. This equivalent capacitance will be referred to as "C.sub.v ". As shown in FIG. 1, the ceramic resonator 10 can be expressed by an equivalent circuit consisting of a parallel capacitance Ca, a series capacitance Cr, a series inductance L and a series resistance r. Therefore, the oscillation frequency f.sub.o obtained from the output terminal 9 can be expressed by the following formula (1): ##EQU1##
The oscillation frequency f.sub.o of the VCO is thus controlled by changing the equivalent capacitance Cv of the variable reactance circuit 11 (i.e., by the control voltage applied to the control terminal 12). Moreover, the variable reactance circuit 11 is designed in such a manner that is capcitance Cv becomes zero during the
free-running oscillation of the VCO. In other words, at the time when no control voltage is applied to the control terminal 12, the free-running oscillating frequency of the VCO is automatically determined at the resonance frequency inherent to the ceramic resonator 10. Thus, the adjustment of the free-running oscillating frequency is unnecessary by employing the resonator 10 whose resonance frequency is the above-mentioned center frequency. Here, it is noted that the capacitance Cv of the variable reactance circuit 11 is designed at zero when no control voltage is applied but is designed so as to change from a negative value to a positive value by the control voltage.
Thus, the VCO shown in FIG. 1 does not require the adjustment of the free-running oscillating frequency. However, this VCO frequently oscillates at an abnormal oscillation frequency which is locked at a transient time such as at a power switching-on time.
More specifically, the ceramic resonator 10 has structurally two resonant frequencies, as seen from the impedance-to-frequency characteristic shown in FIG. 2. That is, the VCO oscillates stably at both of a first frequency f.sub.1 of a lower resonant frequency and a second frequency f.sub.2 in an upper resonant frequency. The second frequency f.sub.2 is as large as about 10 times
the first frequency f.sub.1. As is understood from the formula (1), when the equivalent capacitance component Cv of the variable reactance circuit 11 approaches the parallel capacitance Ca, the oscillation frequency f.sub.o becomes higher to approach the second frequency f.sub.2. In normal operation, the variable range of the equivalent capacitance Cv of the variable reactance circuit 11 is controlled so that it does not approach the parallel capcitance Ca. However, at the transient response time such as switching-on the power, this control does not operate sufficiently, so that the VCO is locked to oscillate at the oscillation of the second frequency f.sub.2.
Moreover, the oscillation loop including the ceramic resonator 10 does not often operate, so that the oscillation inherent to the circuit structure of the amplifier 8 and the variable reactance circuit 11 occurs. This oscillation frequency is lower than the frequency f.sub.1.
In any of these cases, the VCO does not oscillate at the desired free-running oscillation frequency, so that the PLL circuit does not operate.